Receiver stomp-and-restart in a distributed MU-MIMO system using RSSI separation

ABSTRACT

A system and method are provided for performing stomp-and-restart techniques in distributed MU-MIMO system. A plurality of radio head devices are provided that are configured to be deployed separated from each other in a coverage region of interest of a wireless network. A central processor subsystem is provided that is in communication with the plurality of radio head devices. The central processor subsystem configured to perform several operations based on downconverted samples received from the plurality of radio head devices.

TECHNICAL FIELD

The present disclosure relates to wireless communications.

BACKGROUND

In a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)wireless communication system or network, such as an IEEE 802.11wireless local area network, co-channel frame collisions are inevitable.To maximize throughput, the receiver of a wireless device (e.g., anaccess point or a client device) needs to successfully decode one of theframes in the collision.

There are techniques to maximize the probability of successfullydecoding one of the frames involved in the collision. One technique isreferred to as “stomp-and-restart” in which if, after a receiver hasacquired a frame (start-of-packet or “SOP”) and begins decoding thedata, a subsequent stronger frame is received. The receiver will abortdecoding the first frame and re-acquire on the second frame. If thesignal strength of the second frame is much larger than that of thefirst frame, the second frame is likely to be successfully decoded. Itis possible that the second frame is not substantially stronger than thefirst frame, in which case both frames are lost.

Conventional stomp-and-restart mechanisms assume collocated receiverpaths (e.g., multiple-input multiple-output (MIMO)) are impacted inroughly the same way by the colliding packet; that is, all collocatedreceiver paths are subject to the same wideband shadow fading (oneantenna is not blocked by a wall while the other antennas see a clearline-of-sight) and roughly the same path-loss (receive signal strengthvariations across antennas exists, but are relatively small forcollocated antennas). Here, “collocated” refers to antennas spaced onthe order of a wavelength, and remote receiver devices (each comprisinga plurality of antennas) are spaced on the order of ten or a hundred ora thousand wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of distributed multi-user multiple-inputmultiple-output (MU-MIMO) system employing stomp-and-restart techniquesusing receive signal strength separation, according to an exampleembodiment.

FIG. 2 is a more detailed block diagram of the system shown in FIG. 1,according to an example embodiment.

FIGS. 3A and 3B are diagrams illustrating examples of receive signalstrength information for an example described in connection with FIG. 1,according to an example embodiment.

FIG. 4 is a diagram depicting an example of a situation in which thestomp-and-restart techniques presented herein may be employed, accordingto an example embodiment.

FIG. 5 is a flow chart depicting a process for grouping radio headdevices, shown in FIG. 1, into disjoint sets, according to an exampleembodiment.

FIG. 6 is a flow chart depicting a process for stomp-and-restarttechniques in a distributed MU-MIMO system, according to an exampleembodiment.

FIG. 7 is a flow diagram that illustrates signal processing according toanother example embodiment.

FIG. 8 is a diagram illustrating the relationship between null spaceinformation computed in the signal processing flow of FIG. 7, accordingto an example embodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

A system and method are provided for performing stomp-and-restarttechniques in distributed MU-MIMO system. A plurality of radio head(“receiver”) devices are provided that are configured to be deployedseparated from each other in a coverage region of interest of a wirelessnetwork. Each receiver device includes a plurality of antennas and adownconverter associated with each antenna configured to downconvertreceived signals transmitted by two or more client devices, eachdownconverter configured to produce downconverted samples of thereceived signals at each of the plurality of antennas. A centralprocessor subsystem is provided that is in communication with theplurality of receiver devices. The central processor subsystemconfigured to perform several operations based on downconverted samplesreceived from the plurality of receiver devices. After start-of-packetdetection of a first packet by one or more of the plurality of receiverdevices based on the downconverted samples, the central processorsubsystem groups receiver devices into disjoint sets based onmeasurements related to signal strength or signal-to-interference ratioof the downconverted samples associated with the first packet. Thecentral processor subsystem decodes the first packet using downconvertedsamples received by a first set of receiver devices, and determines thata stronger second packet is being received by a particular set ofreceiver devices while the first packet is being decoded when thestronger second packet is detected on any antenna of the particular setof receiver devices. The central processor subsystem continues to decodethe first packet using downconverted samples from the first set ofreceiver devices but without downconverted samples from the particularset of receiver devices associated with the stronger second packet, anddecodes the second packet using downconverted samples from receiverdevices in the particular set of receiver devices and usingdownconverted samples from any one or more additional antennas of areceiver device that detected the second packet.

Example Embodiments

With reference made to FIG. 1, a wireless network system 10 is shownthat includes a central media access control/PHY processor subsystem 20connected to or in communication with a plurality of radio heads (RHs)30(1)-30(8) physically separated from each other and arranged towirelessly communicate with one or more wireless client devices, e.g.,clients 40(1)-40(3). The RHs are also referred to herein as “receiverdevices” though it is to be understood that they may also have transmitcapabilities. While FIG. 1 shows 8 RHs and 3 clients, this is only anexample and the numbers of RHs and clients may be different in an actualwireless network deployment.

As described in more detail hereinafter in connection with FIG. 1, eachRH includes multiple antennas, a downconverter for each antenna, anupconverter for each antenna, an automatic gain controlcircuitry/functionality. As a result, each RH can generate multiplereceive signals, one detected by each antenna, associated with atransmission made by a client device.

With the central MAC/PHY processor subsystem and distributed RHs, theability to receive all of a frame that collides (overlaps in time) withanother frame is greatly improved due to physical separation of the RHsand the large variation in path loss that results. The RHs serve only todetect the start of a frame e.g., an 802.11 physical layer convergenceprocedure (PLCP) protocol data unit (PPDU), perform automatic gaincontrol (AGC) to the signal, and downconvert the received signal tobaseband (or other intermediate frequency) to produce fixed-pointin-phase/quadrature (I/Q) samples. The I/Q samples associated with eachsignal detected by each antenna of a RH are sent over a fronthaulnetwork to the central MAC/PHY processor subsystem 10. In addition, RHsshare a clock and are RF synchronized. The combination of the centralMAC/PHY processor subsystem 20 and the plurality of RHs 30(1)-30(N) forma distributed multi-user-MIMO system when communicating with multipleclient devices simultaneously, as described further hereinafter.

Reference is now made to FIG. 2. FIG. 2 shows the RHs and the centralMAC/PHY processor subsystem in more detail, according to an exampleembodiment. For generality, a plurality of RHs 30(1)-30(N) is shownconnected to the central MAC/PHY processor subsystem 20 by a fronthaulnetwork 50.

Each RH includes a plurality of antennas 32(1)-32(K), a correspondingplurality of downconverters 34(1)-34(K), a corresponding plurality ofupconverters 36(1)-36(K) and a corresponding plurality of AGC blocks38(1)-38(K) (or one AGC block shared across all of the antenna paths).Each downconverter 34(1)-34(K) generates downconverted I/Q samples of asignal received at its associated antenna, which are sent over thefronthaul network 50 to the central MAC/PHY processor subsystem 20. Eachupconverter 36(1)-36(K) upconverts I/Q transmit signals (received fromthe central MAC/PHY processor subsystem 20) for transmission via itsassociated antenna. An analog-to-digital converter (ADC) may be includedin or associated with each downconverter 34(1)-34(K), though not shownin FIG. 2 for simplicity. Likewise, a digital-to-analog converter (DAC)may be included in or associated with each upconverter 36(1)-36(K).

In addition, each RH may include a processor 39 that is coupled to eachof the downconverters 34(1)-34(K) to perform signal processing on thedownconverted samples produced by the downconverters. The processor 39may be a digital signal processor, an application specific integratedcircuit (ASIC) including digital logic gates, a programmable digitallogic, or a microprocessor or microcontroller that executes instructionsstored in memory to carry out various signal processing operations.

The central MAC/PHY processor subsystem 20 includes a bank of basebanddemodulators 60(1)-60(P), a bank of baseband modulators 62(1)-62(P), aMAC processor 64, a control processor 66, and memory 68. The MACprocessor 64 and control processor 66 are each connected to the bank ofbaseband demodulators 60(1)-60(P) and to the bank of baseband modulators62(1)-62(P). The baseband demodulators 60(1)-60(P) perform basebanddemodulation processing on the downconverted I/Q samples received fromeach antenna path of each RH. The baseband modulators 62(1)-62(P)perform baseband modulation processing to generate I/Q transmit signalsto be supplied to each upconverter of each antenna path of each RH. TheMAC processor 64 performs media access control processing. The controlprocessor 66 performs a variety of other functions, including thecentralized stomp-and-restart processing described in greater detailbelow, by executing instructions associated with the control software 70stored in memory 68. In one form, one or more of the basebanddemodulators, baseband modulators, MAC processor 64 and controlprocessor 66 may be implemented in one or more ASICs, digital signalprocessors, programmable digital logic gates, etc.

The memory 68 may include read only memory (ROM), random access memory(RAM), magnetic disk storage media devices, optical storage mediadevices, flash memory devices, electrical, optical, or otherphysical/tangible memory storage devices. Thus, in general, the memory68 may comprise one or more tangible (non-transitory) computer readablestorage media (e.g., a memory device) encoded with software comprisingcomputer executable instructions and when the software is executed (bythe processor 65) it is operable to perform the operations describedherein.

Referring now to FIG. 3A, with continued reference to FIG. 1, the RHsmay detect a frame from each client with the characteristics shown inFIG. 3A, as an example. FIG. 3A shows a table listing per RH/per clientreceive signal strength information, wherein the entries with horizontalcross-hatches are the highest receive signal strength information (RSSI)and the entries with the vertical cross-hatches indicates very highsignal-to-interference ratio (SIR) for those client/RH(s). FIG. 3B showsoverall SIR for each client's best RH, by SIR.

As is evident from FIGS. 3A and 3B, from the diversity of path lossesbetween different RHs and client combinations, there are goodopportunities for enough SIR in order to detect start-of-packet andrecover the PPDUs for different clients. Even for clients that arenearby each other, there can be enough of an RSSI difference betweenclients detected at the same RH due to different client antennapatterns, and that will have the same general effect as being separated.For example, in the case of certain smartphone devices, the RF signalemitted from the back of the device is 10+dB stronger than the signalout of the front of the device. Therefore, the benefit to separated RHscan be used for co-located clients as well as separated clients in manycircumstances.

Reference is now made to FIG. 4 that illustrates an example scenario inwhich the stomp-and-restart techniques presented herein may be employed.When Client[1] at 40(1) transmits PPDU[1], which is detected by multipleRHs (e.g., RHs denoted RH(m) and RH(n) at reference numerals 30(m) and30(n), respectively), the central MAC/PHY processor subsystem 22 groupsRH antennas into subsets for parallel decoding of PPDU[1]. The centralMAC/PHY processor subsystem 22 could use all available RHs in a singledecode process. However, in the event of the collision with PPDU[2],that single decode process would need to restart on PPDU[2] or continueon PPDU[1], and consequently at most only one PPDU would be recoveredfrom the collision. By splitting the decoding into parallel processes,flexible use of resources for detecting and recovering both PPDUs in thecollision is possible.

Partitioning Antennas into Decoding Subsets

Reference is continued to be made to FIGS. 1, 2 and 4. Each RH forms adefault starting subset of antennas. For the purpose of thisdescription, a radio head is a logical partitioning of antennas, whereeach part could coexist in a single mechanical enclosure, in closeproximity to be considered as a group, or all parts could be enclosed ina single mechanical enclosure. It is a predetermined configuration ofthe system. Each RH is capable of performing common actions such asstart-of-packet (SOP) detection, automatic gain control (AGC), etc. Thatis, each RH, by way of operations performed by the processor 39 in eachRH, may perform SOP detection and the AGC block(s) in each RH mayperformed AGC operations, or these operations are performed by thecentral MAC/PHY processor subsystem 22 acting on the group of signalsdesignated as a single RH.

According to the techniques presented herein, a process 100 is providedfor partitioning antennas into decoding subsets is now described withreference to the flow chart of FIG. 5. After start-of-packet detectionby N radio heads, at 110, the central MAC/PHY processor subsystem 20combines radio heads into disjoint sets called decoding sets or decodingclusters. Each decoding set contains a sufficient number of antennas tosatisfy the maximum number of expected spatial streams in a single PPDU(Nant≧Nss). These sets may be formed by ordering the radio headsaccording to RSSI measurements or signal-to-interference-plus-noise(SINR) measurements and cutting off at Nant, or some other partitioningmechanism.

Usually in an AP with multiple receive paths the timing (symbol timing)is common to all receive paths. The sample indices corresponding to asymbol are the same across all receive paths. When multiple radio headsare used in a set, then symbol alignment should be decided common to allpaths.

The radio heads are assumed to be synchronized with their referenceoscillators locked to a common source. In this case, any one of theradio heads could provide carrier frequency offset (CFO) estimation andcorrections made to that estimate. If the radio heads are not locked,which is possible, then the individual radio heads each have a CFOestimate and correction unique to the respective radio head.

At 120, when multiple RH antennas are added to a subset, the notion of amaster RH may be introduced. Collapsing two RHs (or two subsets of RHs)into a single subset is simplified if one RH is the master.

The master RH may be used for CFO estimation. Digital AGC may be made tothe level of the master RH. In addition, timing and resampling isperformed with respect to the master RH. The use of a master RH is,however, optional.

Partitioning the antennas may be made based on certain heuristics,including RSSI (seek common RSSI), channel condition per subset (meanacross subcarriers), and optimal size of decoding subsets. A minimumsize may be required to ensure successful decoding (with forward errorcorrection greater than some threshold) plus collision margin loss ofsignal-noise-ratio. Collision margin can be a system parameter based ondeployment density/type.

Distributed Stomp-and-Restart Operation

Reference is now made to FIG. 6 for a description of a flowchartdepicting operations for a distributed stomp-and-restart process 200.The situation described in connection with process 200 is one in which,while a first signal (e.g., first packet or first PPDU) is beingreceived (from one of the client devices) and decoded by the centralMAC/PHY processor subsystem, a second signal (e.g., second packet orsecond PPDU) is also received and the second packet may have a strongerreceive signal strength. The central MAC/PHY processor subsystem decodesthe first packet using downconverted samples received by an arbitraryfirst decoding set of RHs. At 210, each decoding set independentlyinitiates and sends a receive restart notification (issued by the SOPfunction running on the processor of a RH) to the central MAC/PHYprocessor subsystem in the event a second stronger signal is detected onany of the decoding set's/cluster's antennas while a first signal isbeing received. The receive restart notification serves to alert thecentral MAC/PHY processor subsystem that a stronger second packet isbeing received by a particular set of RHs while the first packet isbeing decoded. The detection of a second stronger signal could indicatethat a second PPDU is being received, in collision with a first signal(first PPDU). At 220, a RH in a decoding set detecting a stronger secondsignal (RSSI change greater than a threshold) sends a receive abort/AGCunlock condition notification to the central MAC/PHY processor subsystem20.

The central MAC/PHY processor subsystem 20 attempts to continue todecode both the first PPDU and second PPDU using two decoding sets.Specifically, at 230, the central MAC/PHY processor subsystem 20 decodesthe original PPDU using signals from the decoding set that detected theoriginal PPDU but removes from the decoding the signal contributionsfrom RHs that initiated the aborting/unlocked condition. Said anotherway, the central MAC/PHY processor subsystem 20 continues to decode thefirst packet using downconverted samples from the first decoding set ofradio head devices but without downconverted samples from the particularset of radio head devices associated with the stronger second packet.When possible, the central MAC/PHY processor subsystem 20 nulls thesignal contributions associated with the new PPDU when decoding theoriginal PPDU. That is, the MAC/PHY processor subsystem 20 nullsdownconverted samples associated with the second packet when decodingthe first packet.

At 240, the central MAC/PHY processor subsystem 20 decodes the secondPPDU using signals from RHs that initiated the aborting/unlockedcondition, and in addition signals from any RH antennas that detectedthe second PPDU, where such additional signals from RH antennas areselected according to RSSI or SINR. Said another way, the centralMAC/PHY processor subsystem 20 decodes the second packet usingdownconverted samples from radio head devices in the particular set ofradio head devices and using downconverted samples from any one or moreadditional antennas of a radio head device that detected the secondpacket. The receive signals associated with the first PPDU may be nulledwhen decoding the second PPDU to improve SINR of the new PPDU.

In some cases, too few antennas would be available, or the SINR would betoo poor, in which case the central MAC/PHY processor subsystem 20aborts processing on unrecoverable PPDUs.

Receiver Stomp-and-Restart in a Distributed MU-MIMO System Using Nulling

This embodiment attempts solves the same problem but with more signalprocessing that can recover collided PPDUs even without much RSSIseparation.

A subset of RHs detect and start processing a first PPDU (e.g. PPDU[1]).Once the channel estimation portion of the PPDU is complete, the RHsinvolved in receiving the first PPDU also generate/synthesize spatialnulling data associated with the first PPDU, and supply the spatialnulling data the central MAC/PHY processor subsystem for use in nullingthe first PPDU when decoding the second PPDU. In this way, if a secondPPDU is transmitted, it can be detected and decoded at the centralMAC/PHY processor subsystem with a relatively high SINR.

However, since the second PPDU may not be symbol aligned or CFO alignedwith the first PPDU, spatial filtering in the frequency domain (after aFast Fourier Transform (FFT)) may not be effective due to inter-carrierinterference (ICI) and inter-symbol interference (ISI). Therefore, inone embodiment, time domain nulling is more general (nRx×nRx matrixmultiplication). However, the channel may vary across the frequencyband, so time domain nulling can only null the mean channel, and mayonly deliver 5 or 10 dB SINR in many typical channels.

At the opposite extreme, the sequence of (CFO correction+FFT+spatialnulling per subcarrier+IFFT)+CFO re-application provides much moreprecise nulling, but has high latency since the FFT/IFFT has 64 pointsfor a 20 MHz IEEE 802.11 signal, and 128, 256 and 612 points for 40, 80and 160 MHz wide signals, respectively.

Thus, the spatial nulling data may be for use in nulling the firstpacket in the frequency domain or for nulling the first packet in thetime domain.

A further embodiment balances nulling quality with latency is to use CFOcorrection+4point-FFT+spatial nulling per subcarrier+4point-IFFT+CFOre-application where the 4point FFT splits a 20 MHz band into 5 MHzsubbands that can be individually nulled using the average MIMO channelacross the 5 MHz. By using a 2, 4, 8 or 16 orthogonalizing reversibletransform, reasonable nulling quality at tolerable latencies can beachieved.

FIG. 7 illustrates a processing flow 300 for a more specific example ofthis further embodiment. The process 300 is performed by the centralMAC/PHY processing subsystem 20. A receive signal is obtained and at305, a packet is detected from a first user (e.g., first client device)denoted User 1, and at the same time a packet may be detected from asecond user (e.g., a second client device) denoted User 2. At 310,synchronization is performed and the cyclic prefix (CP) is removed fromUser 1, At 315, an ITT is performed. At 320, a channel estimationH_(1,k) is made for User 1 at subcarrier k for K subcarriers.

There are two signal processing paths 330 and 380. Processing path 330is based on the output of operation 305 to decode the packet for User 1,and processing path 380 is performed based on the output of operation320 to decode the packet for User 1. Channel information generated at320 is supplied to processing path 330. Channel information generated byprocessing path 330 is supplied to processing path 380 as describedherein.

Describing processing path 330 first, at 332, a sum is made over Ssubbands (4 or 16) of the channel estimate H_(1,k) to produceA_(1,s)=sum_(kinSubband)H_(1,k) for s=1, . . . S. At 334, the null spaceP_(11,s) for signal contributions associated with the packet from User 1is computed from A_(1,s) for s=1, . . . S. At 335, the received signalis filtered to S subbands using the received signal and the null spaceP_(11,s). At 340, signal contributions associated with User 1 in the Ssubbands are canceled in each of the S subbands by the operationsy_(2,1)=P_(11,1)y_(1,1), . . . , y_(2,S)=P_(11,S)y_(1,S). At 345, a sumis computed across the filtered subbands. At 350, an attempt is made todetect the packet of User 2 such that the output of step 250 isstart-of-packet (SOP) detection indicating of the packet of User 2,denoted SOP₂. At 355, synchronization is performed and the CP is removedfrom. User 2. At 360, an FFT is performed (e.g., a relatively smallnumber of points such as 4 points). At 365, the channel H_(2,k) for User2 is estimated at subcarriers k. The output of step 365 is supplied toboth the decode operation 370 and to the processing path 380. At 370,the packet for User 2 is decoded to output ŝ₂.

The processing path 380 is now described. At 385, the output of step 305is filtered to S subbands. At 390, a sum is made over S subbands (4 or16) of the channel estimate H_(2,k) to produceA_(2,s)=sum_(kinSubband)H_(2,k) for s=1, . . . S. At 400, the null spaceP_(22,s) for signal contributions associated with the packet from User 2is computed from A_(2,s) for s=1, . . . S. At 405, using the output fromoperation 400 and the output of operation 385, signal contributionsassociated with User 2 are canceled in each of the S subbands by theoperations y_(1,1)=P_(22,1)y_(1,1), . . . , y_(1,S)=P_(22,S)y_(1,S). At410, a sum is computed across the filtered subbands. At 415, the CP isremoved from User 1. At 420, an FFT is computed (similar to operation360 described above) and at 425, the packet from User 1 is decoded tooutput ŝ₁.

Reference is now made to FIG. 8. FIG. 8 illustrates the null spaceinformation P_(11,k) and P_(22,k) computed from the channel estimatesH_(1,k) and H_(2,k) for overlapping packets from two client devicesdenoted User 1 and User 2.

In performing the process 300 shown in FIG. 7, the central MAC/PHYprocessor subsystem 20 is configured to process the spatial nulling datato compensate for carrier frequency offset of the first packet withrespect to the second packet using carrier frequency offset correction,a Fast Fourier Transform, or a general Digital Fourier Transform,estimation of spatial nulling per subband comprised of one or moresubcarriers, and a filter bank using coefficients based on DigitalFourier Transform coefficients, wherein the filter bank splits afrequency channel into constituent subbands that are individuallynulled.

In summary, in one form, a system is provided comprising: a plurality ofreceiver devices configured to be deployed separated from each other ina coverage region of interest of a wireless network, each receiverdevice including a plurality of antennas and a downconverter associatedwith each antenna configured to downconvert received signals transmittedby two or more client devices, each downconverter configured to producedownconverted samples of the received signals at each of the pluralityof antennas; and a central processor subsystem in communication with theplurality receiver devices, the central processor subsystem configuredto: receive the downconverted samples from the plurality of receiverdevices; after start-of-packet detection of a first packet by one ormore of the plurality of receiver devices based on the downconvertedsamples, group receiver devices into disjoint sets based on measurementsrelated to signal strength or signal-to-interference of thedownconverted samples associated with the first packet; decode the firstpacket using downconverted samples received by a first set of receiverdevices; determine that a stronger second packet is being received by aparticular set of receiver devices while the first packet is beingdecoded when the stronger second packet is detected on any antenna ofthe particular set of receiver devices; continue to decode the firstpacket using downconverted samples from the first set of receiverdevices but without downconverted samples from the particular set ofradio receiver associated with the stronger second packet; and decodethe second packet using downconverted samples from receiver devices inthe particular set of receiver devices and using downconverted samplesfrom any one or more additional antennas of a receiver device thatdetected the second packet.

In another form, a method is provided comprising: receiving transmittedsignals from two or more client devices at a plurality of receiverdevices separated from each other in a coverage region of interest of awireless network, each receiver device including a plurality ofantennas; producing downconverted samples of received signals at each ofthe plurality of antennas of each receiver device; performingstart-of-packet detection of a first packet by one or more of theplurality of receiver devices based on the downconverted samples; afterstart-of-packet detection of the first packet, grouping receiver devicesinto disjoint sets based on measurements related to signal strength orsignal-to-interference of the downconverted samples associated with thefirst packet; decoding the first packet using downconverted samplesreceived by a first set of receiver devices; determining that a strongersecond packet is being received by a particular set of receiver deviceswhile the first packet is being decoded when the stronger second packetis detected on any antenna of the particular set of receiver devices;continuing to decode the first packet using downconverted samples fromthe first set of receiver devices but without downconverted samples fromthe particular set of receiver devices associated with the strongersecond packet; and decoding the second packet using downconvertedsamples from receiver devices in the particular set of receiver devicesand using downconverted samples from any one or more additional antennasof a receiver device that detected the second packet.

In still another form, one or more non-transitory computer readablestorage media encoded with instructions that, when executed by aprocessor, cause the processor to perform operations comprising:performing start-of-packet detection of a first packet received by oneor more of a plurality of receiver devices based on downconvertedsamples generated by one or more of the plurality of receiver devicesfrom transmit signals received from two or more client devices in awireless network, the plurality of receiver devices being separated fromeach other in a coverage region of interest of the wireless network,each receiver device including a plurality of antennas; afterstart-of-packet detection of the first packet, grouping receiver devicesinto disjoint sets based on measurements related to signal strength orsignal-to-interference of the downconverted samples associated with thefirst packet; decoding the first packet using downconverted samplesreceived by a first set of receiver devices; determining that a strongersecond packet is being received by a particular set of receiver deviceswhile the first packet is being decoded when the stronger second packetis detected on any antenna of the particular set of receiver devices;continuing to decode the first packet using downconverted samples fromthe first set of receiver devices but without downconverted samples fromthe particular set of receiver devices associated with the strongersecond packet; and decoding the second packet using downconvertedsamples from receiver devices in the particular set of receiver devicesand using downconverted samples from any one or more additional antennasof a receiver device that detected the second packet.

In still another form, an apparatus is provided comprising a processorand a network interface unit configured to enable communications over anetwork to communicate with a plurality of receiver devices configuredto be deployed separated from each other in a coverage region ofinterest of a wireless network, each receiver device including aplurality of antennas and a downconverter associated with each antennaconfigured to downconvert received signals transmitted by two or moreclient devices, each downconverter configured to produce downconvertedsamples of the received signals at each of the plurality of antennas.The processor is configured to: perform start-of-packet detection of afirst packet received by one or more of a plurality of receiver devicesbased on downconverted samples generated by one or more of the pluralityof receiver devices from transmit signals received from two or moreclient devices in a wireless network, the plurality of receiver devicesbeing separated from each other in a coverage region of interest of thewireless network, each receiver device including a plurality ofantennas; after start-of-packet detection of the first packet, groupreceiver devices into disjoint sets based on measurements related tosignal strength or signal-to-interference of the downconverted samplesassociated with the first packet; decode the first packet usingdownconverted samples received by a first set of receiver devices;determine that a stronger second packet is being received by aparticular set of receiver devices while the first packet is beingdecoded when the stronger second packet is detected on any antenna ofthe particular set of receiver devices; continue to decode the firstpacket using downconverted samples from the first set of receiverdevices but without downconverted samples from the particular set ofreceiver devices associated with the stronger second packet; and decodethe second packet using downconverted samples from receiver devices inthe particular set of receiver devices and using downconverted samplesfrom any one or more additional antennas of a receiver device thatdetected the second packet.

The above description is intended by way of example only. Although thetechniques are illustrated and described herein as embodied in one ormore specific examples, it is nevertheless not intended to be limited tothe details shown, since various modifications and structural changesmay be made within the scope and range of equivalents of the claims.

What is claimed is:
 1. A system comprising: a plurality of receiverdevices configured to be deployed separated from each other in acoverage region of interest of a wireless network, each receiver deviceincluding a plurality of antennas and a downconverter associated witheach antenna configured to downconvert received signals transmitted bytwo or more client devices, each downconverter configured to producedownconverted samples of the received signals at each of the pluralityof antennas; and a central processor subsystem in communication with theplurality receiver devices, the central processor subsystem configuredto: receive the downconverted samples from the plurality of receiverdevices; after start-of-packet detection of a first packet by one ormore of the plurality of receiver devices based on the downconvertedsamples, group receiver devices into disjoint sets based on measurementsrelated to signal strength or signal-to-interference ratio of thedownconverted samples associated with the first packet; decode the firstpacket using downconverted samples received by a first set of receiverdevices; determine that a stronger second packet is being received by aparticular set of receiver devices while the first packet is beingdecoded when the stronger second packet is detected on any antenna ofthe particular set of receiver devices; continue to decode the firstpacket using downconverted samples from the first set of receiverdevices but without downconverted samples from the particular set ofreceiver devices associated with the stronger second packet; and decodethe second packet using downconverted samples from receiver devices inthe particular set of receiver devices and using downconverted samplesfrom any one or more additional antennas of a receiver device thatdetected the second packet.
 2. The system of claim 1, wherein thecentral processor subsystem is configured to group the receiver devicessuch that each decoding set has a sufficient number of antennas toprocess a maximum number of expected spatial streams in a single packetunit transmitted by one or more of the client devices.
 3. The system ofclaim 2, wherein the central processor subsystem is configured to, ingrouping the receiver devices, order the receiver devices according toreceive signal strength or signal-to-interference ratio of downconvertedsamples obtained from the respective receiver devices.
 4. The system ofclaim 1, wherein the central processor subsystem is configured todetermine that the stronger second packet is being received based on areceive restart indication sent by a receiver device in the particularset of receiver devices.
 5. The system of claim 1, wherein one or morereceiver devices that detect the second packet with a receive signalstrength greater than a threshold are configured to send a receiveabort/unlock condition notification to the central processor subsystem.6. The system of claim 5, wherein the central processor subsystem isconfigured to decode the second packet using downconverted samples fromthe one or more receiver devices that sent the receive abort/unlockcondition notification.
 7. The system of claim 1, wherein the centralprocessor subsystem is configured to null downconverted samplesassociated with the second packet when decoding the first packet.
 8. Thesystem of claim 1, wherein the central processor subsystem is configuredto select downconverted samples for the one or more additional antennasbased on receive signal strength or signal-to-interference ratio.
 9. Thesystem of claim 1, wherein the central processor subsystem is configuredto null downconverted samples associated with the first packet whendecoding the second packet.
 10. The system of claim 1, wherein thereceiver devices in the first set of receiver devices are configured togenerate spatial nulling data associated with the first packet and tosupply the spatial nulling data to the central processor subsystem foruse in nulling the first packet when decoding the second packet.
 11. Thesystem of claim 10, wherein the spatial nulling data is for use innulling the first packet in a frequency domain, or for nulling the firstpacket in a time domain.
 12. The system of claim 10, wherein the centralprocessor subsystem is configured to further process the spatial nullingdata to compensate for carrier frequency offset of the first packet withrespect to the second packet using carrier frequency offset correction,a Fast Fourier Transform, or a general Digital Fourier Transform,estimation of spatial nulling per subband comprised of one or moresubcarriers, and a filter bank using coefficients based on DigitalFourier Transform coefficients, wherein the filter bank splits afrequency channel into constituent subbands that are individuallynulled.
 13. A method comprising: receiving transmitted signals from twoor more client devices at a plurality of receiver devices separated fromeach other in a coverage region of interest of a wireless network, eachreceiver device including a plurality of antennas; producingdownconverted samples of received signals at each of the plurality ofantennas of each receiver device; performing start-of-packet detectionof a first packet by one or more of the plurality of receiver devicesbased on the downconverted samples; after start-of-packet detection ofthe first packet, grouping receiver devices into disjoint sets based onmeasurements related to signal strength or signal-to-interference ratioof the downconverted samples associated with the first packet; decodingthe first packet using downconverted samples received by a first set ofreceiver devices; determining that a stronger second packet is beingreceived by a particular set of receiver devices while the first packetis being decoded when the stronger second packet is detected on anyantenna of the particular set of receiver devices; continuing to decodethe first packet using downconverted samples from the first set ofreceiver devices but without downconverted samples from the particularset of receiver devices associated with the stronger second packet; anddecoding the second packet using downconverted samples from receiverdevices in the particular set of receiver devices and usingdownconverted samples from any one or more additional antennas of areceiver device that detected the second packet.
 14. The method of claim13, wherein determining that the stronger second packet is beingreceived is based on a receive restart indication sent by a receiverdevice in the particular set of receiver devices.
 15. The method ofclaim 14, further comprising the one or more receiver devices thatdetect the second packet with a receive signal strength greater than athreshold are configured to generate a receive abort/unlock conditionnotification.
 16. The method of claim 15, wherein decoding the secondpacket comprises decoding the second packet using downconverted samplesfrom the one or more receiver devices that generated the receiveabort/unlock condition notification.
 17. The method of claim 13, furthercomprising nulling downconverted samples associated with the secondpacket when decoding the first packet, and nulling downconverted samplesassociated with the first packet when decoding the second packet. 18.One or more non-transitory computer readable storage media encoded withinstructions that, when executed by a processor, cause the processor toperform operations comprising: performing start-of-packet detection of afirst packet received by one or more of a plurality of receiver devicesbased on downconverted samples generated by one or more of the pluralityof receiver devices from transmit signals received from two or moreclient devices in a wireless network, the plurality of receiver devicesbeing separated from each other in a coverage region of interest of thewireless network, each receiver device including a plurality ofantennas; after start-of-packet detection of the first packet, groupingreceiver devices into disjoint sets based on measurements related tosignal strength or signal-to-interference ratio of the downconvertedsamples associated with the first packet; decoding the first packetusing downconverted samples received by a first set of receiver devices;determining that a stronger second packet is being received by aparticular set of receiver devices while the first packet is beingdecoded when the stronger second packet is detected on any antenna ofthe particular set of receiver devices; continuing to decode the firstpacket using downconverted samples from the first set of receiverdevices but without downconverted samples from the particular set ofreceiver devices associated with the stronger second packet; anddecoding the second packet using downconverted samples from receiverdevices in the particular set of receiver devices and usingdownconverted samples from any one or more additional antennas of areceiver device that detected the second packet.
 19. The non-transitorycomputer readable storage media of claim 18, wherein the instructionsfor determining that the stronger second packet is being received isbased on a receive restart indication sent by a receiver device in theparticular set of receiver devices.
 20. The non-transitory computerreadable storage media of claim 18, wherein the instructions fordecoding the second packet comprise instructions for decoding the secondpacket using downconverted samples from the one or more receiver devicesthat sent a receive abort/unlock condition notification indicating thatthe one or more receiver devices detected the second packet with areceive signal strength greater than a threshold.